This invention relates generally to interferometers and more particularly to improvements in plane mirror interferometer systems.
In ultraprecision positioning systems employing laser interferometers, especially in ultraprecision machining utilizing numerical control and employing multiple-axis slides whose position must be monitored very precisely for ultraprecision control, it is common practice to employ a plane mirror, double-pass laser interferometer system to monitor the position of the moving parts of interest. Plane mirror reflector surfaces, as opposed to cube-corner retroreflectors commonly used in linear interferometer systems, offer the advantage of a very high tolerance for angular misalignment of the beam-reflecting plane mirror surface relative to the axis of the beam.
In a typical plane mirror interferometer arrangement utilizing a dual beam technique, a dual frequency beam (f.sub.1 and f.sub.2) is directed to a remote interferometer housing wherein a polarized beam splitter splits the beam into separate beam paths for frequencies f.sub.1 and f.sub.2. The beams f.sub.1 and f.sub.2 are orthogonally polarized so that f.sub.2 passes through the beam splitter and strikes a fixed reference cube corner mounted with the interferometer housing. The beam of frequency f.sub.1 is reflected at 90.degree. to the entering path through a quarter-wave plate exiting the interferometer housing whereupon it strikes a plane mirror reflector mounted on the object being monitored. The beam is reflected back on itself through the quarter-wave plate causing the polarization of the return frequency to be rotated 90.degree. so that f.sub.1 now passes through the polarizing beam splitter striking another cube corner whereupon the frequency f.sub.1 is retroreflected to again pass through the beam splitter. The beam f.sub.1 is again reflected back upon itself through the quarter-wave plate whereupon its polarization is again rotated 90.degree.. At this point the beam is placed into interference with the beam of frequency f.sub.2 at the polarized beam splitter surface and is reflected out of the interferometer housing along with the beam f.sub.2 passing through the beam splitter.
This beam made up of frequency f.sub.2 -f.sub.1 .+-.2.DELTA.f.sub.1. The change in frequency f.sub.1 is due to a Doppler frequency shift which is above or below f.sub.1, depending upon the direction of displacement of the plane mirror surface relative to the interferometer housing. This composite beam is sensed by a measuring photodetector located to view the interference pattern of the resulting composite beam. Relative motion between the plane mirror surface and the interferometer housing along the measuring beam axis causes a difference in the Doppler shifts in the return frequencies, thus creating a difference between the frequency seen by the measuring photodetector and that of a reference photodetector placed to view an interference pattern of the frequencies f.sub.1 and f.sub.2 emanating from the laser source. This difference is monitored by a subtractor and accumulated in a fringe-count register.
A digital calculator may be used to sample the accumulated value at fixed sampling periods and perform a two-stage multiplication, one for refractive index corrections and the other for conversion to inches or millimeters. The resulting value then updates a display which is programmed to remove the double Doppler effect (.+-.2.DELTA.f.sub.1) to obtain the proper display of the measured displacement relative to .+-..DELTA.f.sub.1. The term "double-pass" refers to the number of times that the beam strikes each surface of interest. For example, with a conventional linear interferometer using a cube corner reflector at the moving surface of interest, the beam from a dual-beam source is split at the surface of a polarizing beam splitter, with one frequency f.sub.2 reflected to the reference cube corner mounted in the housing. The other frequency (f.sub.1) is transmitted to the measurement retroreflector (cube corner) striking the surface of interest only once. Both frequencies are then placed in interference at the polarizing beam splitter surface and reflected back along a common axis to a photodetector as described above.
By double passing the beam to the plane mirror surface of interest, the beam frequency f.sub.1 is changed by twice the relative motion between the interferometer housing and the surface of interest, doubling the resolution of the interferometer since each surface is struck twice. Further, this eliminates errors due to slight tilts of the plane mirror surface relative to the axis of the beam f.sub.1.
Although the addition of the above-described plane mirror interferometer with its improved accuracy for XY coordinate positioning of large or poor quality machine slides or tables with relatively inaccurate positioning drives, etc., there remains the problem of direct measurement of relative displacement between two opposed surfaces. The need for this type of measurement is necessary in a number of machining applications as well as various metrology applications. It has been the practice in the art to use an opposed head configuration of interferometers with subsequent subtraction of signals to calculate the relative displacement measurements. Such setups are costly since two complete optical trains are required for each leg of the measurement. A decrease in accuracy also results since in effect two separate, albeit simultaneous, measurements are performed and numerically subtracted. Due to geometry and stiffness, errors can result from deformations and displacement between the two interferometers. In general, careful alignment is required to guarantee that the two interferometers measure along the same line of action or a cosine error will result. Where space is a premium, two measuring legs can be too cumbersome.